The present invention deals with a method of reconstructing a projection of an object produced from an unknown position in space from a series of projections bearing a known geometric relationship to each other and more particularly it deals with the use of gray-level standard deviations in corresponding subtraction images as similarity measures in order to determine restrospectively the projection angle of a radiograph of interest with respect to the set of basis projections with an iterative coordinate estimation procedure incorporating this technique.
The performance of subtraction radiography is critically dependent on the ability to produce two radiographic images that have close to identical projection geometries. The requirement of identical projections can be satisfied by using a bite-block permitting mechanically rigid and reproducible connections between x-ray source, patient, and film at successive exposures. However, this method is inconvenient for both the patient and the operator, because it requires customized fabrication for each patient and thus is costly and time consuming, and it precludes using radiographs other than those made with that particular bite-block. As a result, the use of subtraction radiography has been restricted to small groups of patients in research applications.
An alternative solution has been proposed by R. L. Webber, U. E. Ruttimann, R. A. J. Groenhuis, and P. Edholm in an article entitled "Synthesis of Arbitrary X-Ray Projections from a Finite Number of Existing Projections" published in SPIE Vol. 535, pp. 84-91, 1985. By using tomosynthesis as discussed by R. A. J. Groenhuis, R. L. Webber, and U. E. Ruttimann in their article entitled "Computerized Tomosynthesis of Dental Tissues" published in Oral Surg., Oral Med., Oral Path., Vol. 56, pp. 206-214, 1983, they have reconstructed a desired arbitrary radiographic projection from multiple projections bearing a known geometric relationship to each other. This method could be used to synthesize the proper projection required for accurate substraction without the need for physically coupling x-ray source, patient, and film.
Assuming then that a basis set of images suitable for tomosynthesis is available, what is required is the location of the x-ray source position that was used to make a particular radiographic projection of interest relative to this basis set.